Allow openvino-nightly

[helena-intel]

OpenVINO recently added nightly releases: https://pypi.org/project/openvino-nightly/
This currently doesn't work well with optimum-intel, because import_utils expects a package named "openvino". This PR fixes that.

$ python -c "from optimum.intel import OVModelForCausalLM; OVModelForCausalLM.from_pretrained('hf-internal-testing/tiny-random-gpt2', export=True, compile=False)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/helena/venvs/openvino_env/lib/python3.10/site-packages/optimum/intel/utils/dummy_openvino_objects.py", line 70, in from_pretrained
    requires_backends(cls, ["openvino"])
  File "/home/helena/venvs/openvino_env/lib/python3.10/site-packages/optimum/intel/utils/import_utils.py", line 264, in requires_backends
    raise ImportError("".join(failed))
ImportError: 
OVModelForCausalLM requires the openvino library but it was not found in your environment. You can install it with pip:
`pip install openvino`. Please note that you may need to restart your runtime after installation.

Note that there is no guaruantee that openvino-nightly works well; it is prerelease software for testing only (see the note on the PyPI page). The only thing that this PR does is make sure that we can use openvino-nightly for testing optimum-intel.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[echarlaix]

thanks @helena-intel !